TV MA is basically the equivalent to R for Television.
Within MA, you get all the different letters for Language, Suggestive Dialogue, Violence, Sexual Situations and whatnot. I am sure it makes it more lax for the TV Networks in regard to how they label their movies, but I think it is a fairly good system.
Kids these days are exposed to a ton of things, and it is really up to parents to monitor what they do and watch. I think that TV gives you pretty good indicators to work with.. if it's TV MA - LVS, you know it's going to be a pretty violent movie with some sexual situations and it may not be appropriate for a younger audience. Plus, on these major networks, a lot of the stuff gets edited out anyways.